Recent work has revealed the existence of a novel class of Zn enzymes, in which the Zn appears to function to activate a coordinated thiolate toward nucleophilic attack. X-ray absorption spectroscopy is Hbeing used to study three such enzymes that are involved in various Haspects of methyl transfer: cobalamin-dependent and cobalamin-independent methionine synthase and cobamide: coenzyme M methyl transferase. EXAFS measurements on the native enzyme, on site directed mutants, and on enzyme samples with a variety of substrates and substrate analogs are in progress, with the goal of elucidating the three dimensional geometry of the Zn active sites.